1. Field of the Invention
This invention relates to an avalanche photodiode quenching circuit for use in photon counting measurements.
2. Discussion of Prior Art
Photon counting measurements were originally, and are to some extend presently, carried out using photomultiplier tubes for photon detection. A typical photomultiplier is however relatively fragile, bulky and expensive. The search for a more convenient alternative has led to the use of photodiodes operated in the so-called avalanche Geiger mode. This mode entails reverse-biasing the photodiode with a bias voltage typically a few volts greater than the photo-diode breakdown voltage V.sub.BR. V.sub.BR is the voltage at which a single photon absorption produces complete electrical breakdown of the photodiode active region by cascaded collision ionisation. It is analogous to the ionisation processes occurring in the gas phase in a Geiger-Muller tube.
Avalanche photodiodes are comparatively inexpensive and rugged, and exhibit high quantum efficiencies. They are not however without disadvantages. In particular, for the purpose of achieving high quantum efficiencies, it is necessary to operate at reverse voltages at least bordering on that capable of producing a self-sustaining avalanche in the photodiode. If the photodiode avalanche current reaches a value referred to as I.sub.latch, typically 50 microamps, the avalanche is self-sustaining in the absence of further photons. This may produce catastropic failure. The photodiode is substantially insensitive to photons while in the avalanche condition. Furthermore, it experiences temperature stress which, after the avalanche is terminated by removing the bias voltage, manifests itself as an increased dark current in subsequent operation. This reduces measurement accuracy and sensitivity, since dark current counts must be subtracted from the total count in a measurement, and both are subject to Poissonian statistics. Furthermore, a sustained current through the photodiode in excess of I.sub.latch tends to fill normally empty defect sites or traps in the photodiode semicondcutor material. These traps have long life times compared to the minimum time between counts or dead-time of the photodiode. Trapped charge carriers are therefore released considerably later than, but are correlated with, a photon absorption responsible for the avalanche creating them. The release produces so-called after pulses which are detected by the measuring circuitry monitoring the photodiode. This is a serious problem in the field of photon correlation spectroscopy in particular, since it means that the detection system introduces a degree of correlation between detected pulses which is absent in the original light beam. The measured autocorrelation function will therefore exhibit spurious features which affect or even invalidate the measurement results.
To circumvent these difficulties, the approach in the art has been to provide means for quenching an avalanche as soon as possible after initiation and detection. One particularly simple approach is referred to as passive quenching. It involves arranging the photodiode in series with a comparatively large series resistor, e.g. 220 kohm, and applying the bias voltage across the series arrangement. Prior to photon absorpiton, i.e. when the photodiode is quiescent, the bias appears across the substantially non-conducting photodiode. After absorption, the resistor limits the miximum current taken by the photodiode to a value below I.sub.latch when the falling voltage across the photodiode becomes equal to V.sub.BR. The avalanche is therefore automatically terminated. This arrangement is adequate for comparatively low photodetection rates up to 250 KHz and light intensity fluctuation frequencies up to the same value. However, its disadvantage is that the photodiode is comparatively slow to recover from a detection event. The photodiode must recharge its capacitance through the large resistance before it returns to the quiescent or photosensitive state and this leads to a dead-time in the order of 1 microsecond. Furthermore, during recharge, the photodiode has a variable and increasing sensitivity, so that the dead-time is ill-defined.
Dead-time limitations render the passively quenched avalanche photodiode suitable for photon correlation laser anemometry and spectroscopy experiments where the photon correlator sample time or delay is greater than a few microeconds. However, light intensity fluctuation frequencies greater than 1 MHz regualarly occur in photon correlation measurements on particle diameters of a few tens of nanometres, and also in transonic and supersonic fluid flow measurements by laser Doppler anemometry. A passively quenched avalanche photodiode is not capable of detecting such frequencies.
In IEEE Transactions on Nuclear Science, Vol NS-29, No 1, Feb. 1982 (Reference 1), Cova et al describe active quenching circuits for an avalanche photodiode. In this technique, an avalanche is detected very quickly after initiation. A feedback circuit responds by applying a quenching pulse to the photodiode, taking its reverse bias voltage below breakdown and quenching the avalanche. After quenching, a reset pulse is applied to the photodiode to restore its reverse voltage to the original above-breakdown value. The photodiode is accordingly both actively quenched and actively reset. This produces a very short dead-time in the order of a few tens of nanoseconds. However, in practice this technique possesses disadvantages. The photodiode has a reverse voltage of about 4 V in excess of its breakdown voltage V.sub.BR, and it is required to detect an avalanche as soon as possible after this voltage has begun to fall. It is necessary for the quenching circuit to respond to a fall of a few tens of millivolts. Moreover, the reset pulse is required to re-establish the original reverse voltage very accurately without re-triggering the feedback circuit and generating a spurious count. In practice this is difficult to achieve. Furthermore, the circuits are characterised by an ill-defined dead-time. Two photon absorption events too close together in time produce a situation in which a counter has not fully recovered from a first pulse before it receives a second, and the second is not detected. This results in discrimination against recordal of second pulses; it is known as the "odd-even" effect, since for example a first or odd-numbered pulse is more likely to be counted than a second or even-numbered pulse. In a typical photon correlator, this will introduce spurious correlation effects distorting the measured correlation function.